It is desirable that a power amplifier used in a transmitter such as one used in a base station device used for a wireless communication has highly power-efficient. However, because the linearity and the power efficiency of a power amplifier are characteristics that are in trade-off relationship, when the power amplifier is used in a high linearity region, the power efficiency is reduced. Thus, to enable the power amplifier to operate highly efficiently, the power amplifier is used in a low linearity region.
Because the quality of transmission signals degrades when an amplifier is operated in a low linearity region near the saturation region, as a technology for compensating for a distortion in the amplifier for a transmitter, predistortion, for example, has been suggested. Predistortion is a method for improving the linearity by adding reverse characteristics of the amplifier beforehand to signals input to the amplifier so that the distortion at the output end of the amplifier is cancelled out. The transmitter is provided with distortion compensation coefficients representing reverse characteristics of those of transmission signals with a certain average power, reads a distortion compensation coefficient correspondingly to the instantaneous amplitude of a transmission signal, and generates a predistortion signal by adding the distortion compensation coefficient to the transmission signal. The transmitter also calculates an update for a distortion compensation coefficient based on the transmission signal and an output signal fed back from the amplifier, and updates the distortion compensation coefficient stored in a distortion compensation coefficient table. The transmitter then executes these processes repeatedly to update the distortion compensation coefficients contained in the distortion compensation coefficient table, and converges the distortion compensation coefficients. In this manner, the transmitter compensates for the distortions in the amplifier, and suppresses the degradation of the signal quality. Related-art examples are described in Japanese Laid-open Patent Publication No. 2010-147805.
However, according to the conventional technology, the distortion compensation coefficient is re-calculated when the average transmission power changes. Therefore, depending on the degree of the change in the average power, the transmitter might require some time to converge the distortion compensation coefficients. For example, when the degree of the change in the average power is small, the distortion compensation coefficient hardly changes from the one before the change in the average power. Therefore, the number of times the distortion compensation coefficient updating operation is performed can be reduced. Thus, the time required to converge the distortion compensation coefficients is reduced as well. On the contrary, if the average power changes by a large degree, the distortion compensation coefficients will be changed by a large degree. Therefore, the time required to converge the distortion compensation coefficients will be extended.